Sales of certified used vehicles slipped in June for the first time in a year and a half. But analysts still predict certified sales will rise overall this year.

Sales dipped 2 percent in June from a year earlier to 181,112, but still posted an 8 percent gain for the first half, to 1,133,588. It was the first monthly drop since a 1 percent decline in December 2012.

Larry Dixon, an analyst at NADA Used Car Guide, says any number of factors could have affected certified sales in June, from intenders switching to new vehicles to a shortage of off-lease vehicles available from manufacturers whose output was disrupted in 2011 by the earthquake and tsunami in Japan.

"Long term, the trend is going to continue to go up for CPO sales," Dixon says.

Joe Spina, remarketing director at Edmunds.com, agrees. His data indicate that certified sales volume as a share of franchised dealers' used-vehicle sales is up one percentage point so far this year. "The trend is still upward for the year," he says.

Much of the certified sales decrease was because of drops at Toyota Motor Sales U.S.A., General Motors and American Honda Motor Co.

Toyota's certified vehicle sales fell 14 percent in June to 33,447, and 4 percent in first six months to 212,533.

In April, Toyota Division told Automotive News that its off-lease volume -- the bread and butter of its certified program -- was expected to fall 21 percent this year from 2013, the result of production cuts related to the 2011 natural disasters. That inventory shortage likely would affect certified sales, it said.

Honda's certified sales dropped 18 percent in June to 21,434 and 4 percent in the first half to 138,356.

GM's certified sales in June slid 5 percent to 30,536, but sales for the first six months rose 8 percent to 195,918.

The June slide came as certified used-vehicle sales of GM's discontinued brands are winding down. In June, certified Pontiac sales tumbled 73 percent to 106 while certified Saturn sales collapsed 82 percent to 35. No certified Hummers were sold. GM discontinued the brands as part of its bankruptcy reorganization in 2009.

Though sales of certified Chevrolet cars and trucks in June dropped 9 percent to 20,717, neither Spina nor Dixon believes it is related to the massive recalls of GM vehicles. Chevy's certified sales for the first six months rose 6 percent to 136,442.